1. Field of the Invention
The present invention relates to an ink jet recording head that records by discharging recording liquid (which may be referred to as ink) from the discharge ports by the utilization of thermal energy to cause ink to adhere to a recording medium, and also, relates to a substrate for use of such head. The invention also relates to an ink jet cartridge and an ink jet recording apparatus. More particularly, the invention relates to a substrate of ink jet recording heads used for an ink jet recording head of the kind, which is provided with the heat generating units arranged for it to generate thermal energy, and also, relates to an ink jet recording head formed by use of such substrate, an ink jet cartridge, and an ink jet recording apparatus as well.
2. Related Background Art
There has been known conventionally the so-called bubble jet recording method, that is, an ink jet recording method whereby to discharge ink from the discharge ports by the utilization of acting force exerted by the abrupt change of states following the creation of bubbles in ink by the application of thermal energy given to ink. In general, the ink jet recording apparatus that adopts this bubble jet recording method uses an ink jet recording head provided with the discharge ports from which ink is discharged; ink paths communicated with the discharge ports; heat generating devices that apply thermal energy to the ink which is distributed in each of ink paths. Each of the heat generating devices is arranged on a silicon substrate formed by means of semiconductor wafer process technologies and techniques. Each of the ink paths is structured by bonding a ceiling plate member having the discharge ports and the grooves which are communicated with the discharge ports formed on this plate with the substrate having the heat generating devices arranged on it after having positioned the heat generating devices and the grooves so as to enable them to face each other.
In accordance with a recording method of the kind, it is possible to record images in higher quality at higher speeds with a lesser amount of noises. At the same time, it becomes possible to arrange the discharge ports of the recording head in higher density. Among many advantages, therefore, this method has a remarkable advantage that with a smaller apparatus, it is easier to obtain recorded images in higher resolution, and in colors as well. As a result, this recording method has been utilized widely in recent years for a printer, a copying machine, a facsimile equipment, and many other office equipments.
Nevertheless, for example, if it is attempted to implement a full-line arrangement in a density higher still, there is a need even for such an ink jet recording apparatus as described above a higher level of technological standard from the viewpoint of the manufacture of recording heads that directly affect the design considerations of its structure, the recording accuracy, and the reliability and durability of the recording head, as well as its productivity and adoptability for the large-scale production. In the specifications of Japanese Patent Laid-Open Application Nos. 57-72867 and 57-72868, there are disclosed ink jet recording heads, each provided with a substrate having on one and the same substrate the heat generating devices, and the functional devices that form various circuits to control the drivers that drive the heat generating devices, and also, control each driving of the heat generating devices.
In this respect, however, since the ink jet recording heads disclosed in the specifications of these Japanese Patent Laid-Open Application Nos. 57-72867 and 57-72868 are structured each by the provision of the heat generating devices and the functional devices used for them on one and the same substrate in order to enhance its integrational structure, respectively. Therefore, the size of each device, the width of each electric wire, and each gap between electric wires should be made comparatively small eventually. Here, if the structure should be arranged as disclosed in the specifications of Japanese Patnet Laid-Open Application No. 60-159060 so that an inorganic insulator is formed as a first protection film on the heat generating devices, and an inorganic material is provided as a second protection film, there tends to occur failure due to the short circuit between the electric wiring members and the second protection film, which may be caused by the defective formation of the first protection film in its film formation process or by the defects or the like that may take place due to membrane stress occurring in the film formation of the second protection film.
Also, for each of the ink jet recording heads disclosed in the specifications of Japanese Patent Laid-Open Application Nos. 57-72867 and 57-72868, a number of heat generating devices and functional devices are formed on a substrate at the same time. As a result, each layer is formed on the substrate repeatedly one after another on this substrate and a part of the layer is removed likewise in the head manufacture processes. Therefore, when the uppermost layer is formed, the surface thereof shows fine irregularities having step wedge portions (stepped portions) thereon. For that matter, the step coverage capability on the uppermost layer becomes very important in consideration of the stepped portions thus existing. In other words, if the step coverage at the stepped portions is unfavorable, ink or other recording liquid tends to be permeated through such portions when the substrate is used as a recording head, and electrolytic corrosion or electric insulation breakage may take place as the case may be. Also, if the probability of the occurrence of such defective portions is not small on the uppermost layer formed due to the way of manufacture of the substrate, recording liquid may be allowed to permeate through them to make the life of heat generating devices and the electric wiring shorter considerably.
In this respect, therefore, it is attempted to provide the first protection film in order to improve the step coverage as to the stepped portions of the second protection film even in a case where each width of wires and the gap between each of them are small. With this arrangement, however, the efficiency of heat transfer is lowered between each of the heat generating devices and the surface of the second protection film. The efficiency of the electrothermal conversion is also lowered. Therefore, to maintain the thermal energy on the surface of the second protection film, it is necessary to increase the voltage applied to each of the heat generating devices to the extent that the thermal energy may be lost by the presence of the first protection film, and compensate such efficiency of heat transfer thus lowered. Here, for the improvement of such efficiencies, the thickness of the protection film formed on each heat generating device may be made as small as possible. With the thinner protection film, however, it becomes difficult not only to maintain the step coverage on the stepped portions, but also, lower the probability of the occurrence of defects at least to the extent that such occurrence may be negligible in practice. Further, from the structural viewpoint of the substrate, at least one layer of insulation film is needed. Also, ink of pH 3 to pH 10 is used depending on its usage. Therefore, the protection film which should be in contact with ink is not allowed to be dissolved with the pH of 3 to 10.
Here, SiO.sub.2 film is often used as the first protection film, because it has a comparatively good mechanical strength, and contactness with the cavitation proof film formed by metallic material such as Ta. However, since the SiO.sub.2 film is dissolved by the strong base alkaline solution, there is a possibility that if the cavitation proof film of Ta or the like should carry some defects, the SiO.sub.2 film may be in contact with ink and dissolved eventually. Then, the Al that forms electrodes is also dissolved. Lastly, the electric breakage may be caused in some cases.
Also, for the same reasons for the adoption of the SiO.sub.2 film as described above, Si.sub.3 N.sub.4 film may also be used as the first protection film. However, since the Si.sub.3 N.sub.4 film is formed by the application of CVD method, the film formation temperature is 300.degree. C. to 400.degree. C., which is comparatively high as compared with the sputtering method. Here, although the Si.sub.3 N.sub.4 film may be formed at a lower temperature of 200.degree. C. to 300.degree. C., its contactness is lowered with the metallic nitride, such as TaN, which is the formation material of the heat generating devices. Now, therefore, if the Si.sub.3 N.sub.4 film should be formed at a temperature of as high as 300.degree. C. to 400.degree. C., the hillocks (extrusions) are developed in the Si.sub.3 N.sub.4 film on the Al layer which is the material of the electrodes. Then, there is a possibility that short circuit is caused to occur with the second protection film which is formed later by metallic material such as Ta.
Further, in other words, when the heat generating devices are driven, liquid on each of them is heated and vaporized by the film boiling thus generated, and then, coagulated instantaneously. As a result, in the vicinity of each of the heat generating devices, foaming and coagulations are repeated at a high frequency of several thousands times per second. Conceivably, therefore, the pressure changes (cavitation and corrosion) cause the substrate to be damaged as the case may be.
Now, meanwhile, the ink jet recording heads of cartridge type having ink tank and head integrally formed for use are sold on the market in a considerable amount recently. For an ink jet recording head of the kind, it should be good enough if only its durability is maintained at least until ink in the ink tank is completely used in this particular case. On the other hand, along with the increasing demands on the ink jet recording heads, it is attempted to develop them so as to be suitably usable in more varied fields. As a result, it becomes necessary for them to use different recording liquids in order to meet the requirement of different uses. As described above, however, the recording liquid should be vaporized, and the heat generating devices should be heated to a high temperature in an extremely short period of time. As a result, the colorant and other components contained in ink are decomposed at its molecular level to become the refractory substances. Then, there is a tendency that such substances adhere to the heat generating devices firmly. If the organic or inorganic refractory substrates are fixed on the heat generating devices firmly, the heat transfer from each of them to recording liquid becomes uneven to make the foaming of recording liquid instable.